1. Field of the Invention
The present invention relates to a method of manufacturing tyres different from each other, of the type comprising the steps of: making a plurality of structural components for tyres under production; manufacturing the tyres by assembling the components of each of them following a pre-established sequence, at respective work stations disposed along a manufacturing line on which tyres being processed are caused to move by sequential transfer of same from each work station to the next work station; transferring the manufactured tyres to a curing line; curing the tyres in respective vulcanization moulds associated with said curing line.
It is also an object of the present invention an apparatus for manufacturing tyres different from each other, of the type comprising: a manufacturing line having a plurality of work stations, each arranged to assemble at least one respective structural component on a tyre being processed, transfer devices operating along the manufacturing line to sequentially transfer each tyre being processed from a work station to the next work station; a curing line having vulcanization moulds for the manufactured tyres.
2. Description of the Related Art
A tyre for vehicle wheels usually comprises a carcass structure essentially consisting of one or more carcass plies substantially having a toroidal conformation and the axially opposite side edges of which are engaged with respective annular reinforcing structures incorporating circumferentially inextensible inserts usually called “bead rings”. Each annular reinforcing structure is incorporated into a so-called “bead” defined along an inner circumferential edge of the tyre for anchoring of the latter to a corresponding mounting rim.
Applied to the carcass structure, at a radially outer position thereof, is a belt structure comprising one or more belt strips in the form of a closed ring, essentially made up of textile or metallic cords suitably oriented with respect to each other and to the cords belonging to the adjacent carcass plies.
In addition, applied to the belt structure, at a radially outer position thereof, is a tread band, usually comprised of a band of elastomer material of appropriate thickness.
It should be pointed out that, as regards the present description, by the term “elastomer material” it is meant a rubber blend in its entirety, i.e. a mixture formed of at least one polymeric base suitably amalgamated with reinforcing fillers and/or process additives of different types.
Applied to the opposite sides of the tyre is a pair of sidewalls, each of which covers a side portion of the tyre included between a so-called shoulder region, placed close to the corresponding side edge of the tread band, and the corresponding bead.
After the above statements, it is to be noted that each tyre model is essentially distinguishable from the others due to a plurality of physico-chemical, structural, dimensional and appearance features.
The physico-chemical features are essentially linked with the type and compositions of the materials, in particular the recipes for the different blends used in making the elastomer materials. The structural features essentially define the number and type of the structural components present in the tyre, and their mutual positioning in the tyre structure. The dimensional features refer to the geometric measurements and the profile in right section of the tyre (outer diameter, chord or maximum width, sidewall height and ratio between the latter, i.e. section ratio) and will be hereinafter merely referred to as “size”. The appearance features consist of the tread pattern, ornamental motifs and different inscriptions or distinguishing marks reproduced on the tyre sidewalls and generally identified as “tread pattern” in the following of the present description.
The traditional production processes essentially contemplate four distinct steps in tyre manufacture: a) preparation of the blends, b) production of the individual structural components, c) assembling of the different structural components in succession to obtain a green carcass, d) curing of the carcass and simultaneous moulding of the tread pattern onto the outer surface of the tyre.
In an attempt to reduce production costs, the developing technology fundamentally addressed itself to the search of technical solutions that would bring to achievement of increasingly quicker and more reliable machinery, so as to minimize time necessary for producing each individual tyre, the quality of the finished product being the same or even better.
Thus plants have been accomplished which have high production capacities in terms of number of pieces produced in the time unit, by adopting manufacturing machinery having reduced possibilities of modification (i.e. capable of producing only a limited variety of tyre models), but adapted to maximize a mass production of tyres having identical structural features. By way of example only, in the most modern plants a productivity of about two carcasses per minute can be reached.
Attempts have been also made for reducing or eliminating storage of the semifinished products obtained between two steps in succession of the four process steps previously listed, so as to minimize costs and involved problems each time the tyre model under production is to be changed. For instance, in document EP 922561 a method of managing tyre production is suggested in which, in order to reduce or eliminate a storage time for green tyres and therefore the number of green tyres being stocked, arrangement of a curing line having a number of moulds suitable to constantly absorb the productivity of the manufacturing line is provided. Production of tyres of different models, in particular of different sizes, is achieved by replacing and/or adapting each time the machinery arranged in the manufacturing line, concurrently with replacement of the moulds in the curing line.
It should be recognized however that, at all events, tyre production involves costs that become increasingly higher as the variety of tyre models to be produced increases: in particular interventions on the processes and/or plants for blend production are necessary in order to enable production of components with new and different physico-chemical features and/or interventions on the production plants for the individual structural components in order to change the size of the tyres being produced. Change of the operating sequence (different assembling methodology) and/or the equipment, and adjustment of the manufacturing machinery is also required each time the structure and/or size of the tyre to be produced has been changed. Finally, availability of at least one vulcanization mould for each different “tread pattern-size” pair is required.
The above involves heavy costs for purchasing moulds of different sizes and different tread patterns, as well as different equipment, costs due to non-saturation of same, losses in productivity due to machine downtime (changing of a process or equipment generally involves machine downtime), and machine shop rejections. For instance, in the case of continuously produced components, due to downtime of the downstream plants and/or change in the component features, an excess in production is generated that often must be discarded, its reuse being impossible.
Under this circumstance, a person skilled in the art aiming at minimizing costs generally advises against production of a great number of tyre models on one and the same plant. In fact the cost-minimizing objective is inconsistent with a frequent change of equipment and production processes. Where sale volumes of each model are rather high, there is a tendency to multiply the number of the productive plants so as to make it possible to continuously produce a different model on each plant, thereby succeeding in minimizing the above drawbacks. On the other hand, where the expected sale volumes for given models are not particularly high, on an annual basis for example, it is at all events preferred to continuously make the whole production of at least one year immediately, for the purpose of controlling the production cost. This system however may adversely affect the quality of the product sold and increases warehouse costs, because the goods are stored for a long period of time. In addition, sale risks are increased due for example to an unexpected quick obsolescence of the product, and there is an increase in financial costs for capital locking up as a result of the product remaining in stock and the non-saturation of the moulds that are only used for the limited period of time required for execution of the estimated reduced volume.
In order to cope with the above problems, the Applicant has already set up a productive method according to which each series of tyres under production, identical with each other, is divided into daily batches each comprising an amount of tyres adapted to fill the daily productivity of the moulds. In this way production of tyres of different sizes and/or different constructional features is optimized, storage of great amounts of green and vulcanized tyres being eliminated. This method is described in the European Patent Application No. EP 875364 to which please refer for further possible information.